This invention relates to air regulating systems for controlling the rate of entry of air and other gases, into and out of electrochemical batteries and cells with gas consuming electrodes, and to the batteries and cells in which such air regulating systems are used, particularly air-depolarized, air-assisted and fuel cells and batteries.
Electrochemical battery cells that use a fluid, such as oxygen and other gases, from outside the cell as an active material to produce electrical energy, such as air-depolarized, air-assisted and fuel cell battery cells, can be used to power a variety of portable electronic devices. For example, air enters into an air-depolarized or air-assisted cell, where it can be used as, or can recharge, the positive electrode active material. The oxygen reduction electrode promotes the reaction of the oxygen with the cell electrolyte and, ultimately, the oxidation of the negative electrode active material with the oxygen. The material in the oxygen reduction electrode that promotes the reaction of oxygen with the electrolyte is often referred to as a catalyst. However, some materials used in oxygen reduction electrodes are not true catalysts because they can be at least partially reduced, particularly during periods of relatively high rate of discharge.
One type of air-depolarized cell is a zinc/air cell. This type of cell uses zinc as the negative electrode active material and has an aqueous alkaline (e.g., KOH) electrolyte. Manganese oxides that can be used in zinc/air cell air are capable of electrochemical reduction in concert with oxidation of the negative electrode active material, particularly when the rate of diffusion of oxygen into the air electrode is insufficient. These manganese oxides can then be reoxidized by the oxygen during periods of lower rate discharge or rest.
Air-assisted cells are hybrid cells that contain consumable positive and negative electrode active materials as well as an oxygen reduction electrode. The positive electrode can sustain a high discharge rate for a significant period of time, but through the oxygen reduction electrode, oxygen can partially recharge the positive electrode during periods of lower or no discharge, so oxygen can be used for a substantial portion of the total cell discharge capacity. This means the amount of positive electrode active material put into the cell can be reduced and the amount of negative electrode active material can be increased to increase the total cell capacity. Examples of air-assisted cells are disclosed in commonly assigned U.S. Pat. Nos. 6,383,674 and 5,079,106.
An advantage of air-depolarized, air-assisted, fuel cells is their high energy density, since at least a portion of the active material of at least one of the electrodes comes from or is regenerated by a gas contained in air from outside the cell.
A disadvantage of these cells is that the maximum discharge rates they are capable of can be limited by the rate at which oxygen can enter the oxygen reduction electrode. In the past, efforts have been made to increase the rate of oxygen entry into the oxygen reduction electrode and/or control the rate of entry of undesirable gases, such as carbon dioxide, that can cause wasteful reactions, as well as the rate of water entry or loss (depending on the relative water vapor partial pressures outside and inside the cell) that can fill void space in the cell intended to accommodate the increased volume of discharge reaction products or dry the cell out, respectively. Examples of these approaches can be found in U.S. Pat. No. 6,558,828; U.S. Pat. No. 6,492,046; U.S. Pat. No. 5,795,667; U.S. Pat. No. 5,733,676; U.S. Patent Publication No. 2002/0150814; and International Patent Publication No. WO02/35641. However, changing the diffusion rate of one of these gases generally affects the others as well. Even when efforts have been made to balance the need for a high rate of oxygen diffusion and low rates of CO2 and water diffusion, there has been only limited success.
At higher discharge rates, it is more important to get sufficient oxygen into the oxygen reduction electrode, but during periods of lower discharge rates and periods of time when the cell is not in use, the importance of minimizing CO2 and water diffusion increases. To provide an increase in air flow into the cell only during periods of high rate discharge, fans have been used to force air into cells (e.g., U.S. Pat. No. 6,500,575), but fans and controls for them can add cost and complexity to manufacturing, and fans, even micro fans, can take up valuable volume within individual cells, multiple cell battery packs and devices.
Another approach that has been proposed is to use valves to control the amount of air entering the cells (e.g., U.S. Pat. No. 6,641,947 and U.S. Patent Publication No. 2003/0186099), but external means, such as fans and/or relatively complicated electronics, can be required to operate the valves.
Yet another approach has been to use a water impermeable membrane between an oxygen reduction electrode and the outside environment having flaps that can open and close as a result of a differential in air pressure, e.g., resulting from a consumption of oxygen when the battery is discharging (e.g., U.S. Patent Publication No. 2003/0049508). However, the pressure differential may be small and can be affected by the atmospheric conditions outside the battery.
Reducing friction between the plates of a sliding plate valve by applying a layer of a polymeric film, such as a polyethylene film on, or some other means of reducing the frictional resistance, such as a silicon oil, between a sliding magnetic valve plate and a stationary plate is disclosed in U.S. Pat. No. 5,554,452. U.S. Patent Publication No. 2005/0136321 discloses the application of materials, such as polymeric coatings or films or liquids such as silicon-based oils, to interfacial surfaces of adjacent valve plates to improve the effectiveness of the sliding plate valve seal in the closed position; the improvement can result from reducing friction, filling in imperfections in the interfacial surfaces, or, when an oil is used, through capillary attraction between the plates. U.S. Patent Publication No. 2009/0081519 discloses the addition of round members to a fluid between the valve plates to further improve the seal effectiveness and operation of a sliding plate valve.
It is desirable to further improve the operation and effectiveness of an air regulating system for controlling the flow of air into a gas consuming cell and overcome disadvantages in previous attempts to do so. It is an object of the invention to provide an air regulating system with a valve having two plates, and capable of providing a seal between the plates when the valve is closed over a long period of time. It is also an object of the invention to provide an air regulating system including a valve that can be well sealed over a long period of time and can be operated by applying only a small force. It is a further object of the invention to provide a battery with a gas consuming electrode and an improved air regulating system for controlling the passage of air into the battery, wherein the air regulating system is effective for a long period of time without adversely affecting battery performance.